Exhaust emissions from motor vehicles are a significant source of air pollution. The most significant vehicular emissions comprise pollutants such as carbon monoxide (CO), oxides of nitrogen (NO, N2O, NO2), unburnt hydrocarbons (HC), oxides of sulfur (SO2), and particulates. For conventional gasoline engines dramatic decreases in tailpipe emissions have been realized by the introduction and refinement of the three-way catalytic converter. Unfortunately, the lean-bum operation of diesel or gasoline direct-injection engines is incompatible with this established technology.
Various techniques have been explored to control diesel emissions, e.g., engine modification and/or the use of exhaust after treatment such as catalytic control systems which eliminate pollutants in the exhaust stream by promoting chemical changes to convert unwanted compounds into more benign species. However, for effective emission control a diesel oxidation catalyst must be active at the comparatively low temperatures (e.g., typically below 550° C.) of the diesel exhaust, i.e., must posses a low temperature “light off” (i.e., a low temperature (e.g., less than 200° C. after aging) at which the catalyst becomes 50% efficient for a particular emission component).
A further area of concern arise from catalyst poisoning, e.g., by sulfur compounds and sintering of the platinum group metal (PGM). Poisoning by sulfur compounds can markedly decrease catalyst activity by affecting the platinum group metal oxidation function by preferential adsorption/site blocking. In addition sulfur compound may also compromise support integrity, e.g., by the chemical modification of the support (such as conversion of Al2O3 into Al2(SO4)3) and poison any oxygen storage function present in the catalyst formulation with further loss in activity.
There continues to be a need for methods and catalysts for use in emissions reductions.